Several semiconductor wafer processes include wafer thinning steps. In some applications, the wafers are thinned down to a thickness of less than 100 micrometers for the fabrication of integrated circuit (IC) devices. Thin wafers have the advantages of improved heat removal and better electrical operation of the fabricated IC devices. In one example, GaAs wafers are thinned down to 25 micrometers to fabricate power CMOS devices with improved heat removal. Wafer thinning also contributes to a reduction of the device capacitance and to an increase of its impedance, both of which result in an overall size reduction of the fabricated device. In other applications, wafer thinning is used for 3D-Integration bonding and for fabricating through-wafer-vias.
In the manufacturing of 3D-integrated semiconductors, two primary methods of assembly are utilized, WLP (wafer level packaging) and C2W (Chip to wafer stacking). In both cases, thinned semiconductors are mass-produced by using a temporary adhesive to bond the device wafer to a carrier wafer (typically glass or silicon) for the purpose of support during thinning and post processing after thinning. The device wafer is thus supported during thinning by the carrier. We refer to this as the primary temporary wafer carrier.
The thinned device wafer on the temporary wafer carrier is further processed to create through-silicon-vias (TSV) which terminate in plated or deposited interconnects on one or both side surfaces of the wafer. The surface structures may include landing bond pads, interconnect metal posts or plated micro bumps to form the primary interconnect for 3D stacking. Device wafers needing structures on both sides may require “flipping” to expose the secondary device wafer surface after release from the primary temporary wafer carrier.
Device wafers thinned below 70-80 micrometers cannot be handled without support. The primary methods of support involve bonding the device wafers to primary carrier wafers using temporary removable adhesives. Following thinning of the device wafer, the primary carrier wafer is removed. A secondary temporary carrier is usually used to support the thinned wafer for further processing. The secondary temporary wafer carrier needs to be portable, needs to be able to provide the required means for support to allow transport of the thinned device wafer to other process stations, and must prevent flexure stress and/or cracking after the device wafer is released from the primary temporary carrier.
Wafer thinning is usually performed via back-grinding and/or chemical mechanical polishing (CMP). CMP involves bringing the wafer surface into contact with a hard and flat rotating horizontal platter in the presence of a liquid slurry. The slurry usually contains abrasive powders, such as diamond or silicon carbide, along with chemical etchants such as ammonia, fluoride, or combinations thereof. The abrasives cause substrate thinning, while the etchants polish the substrate surface at the submicron level. The wafer is maintained in contact with the abrasives until a certain amount of substrate has been removed in order to achieve a targeted thickness.
For wafer thicknesses over 200 micrometers, the wafer is usually held in place with a fixture that utilizes a vacuum chuck or some other means of mechanical attachment. However, for wafer thicknesses of less than 200 micrometer and especially for wafers of less than 100 micrometers, it becomes increasingly difficult to mechanically hold the wafers and to maintain control of the planarity and integrity of the wafers during thinning. In these cases, it is actually common for wafers to develop microfractures and to break during CMP.
An alternative to mechanical holding of the wafers during thinning involves attaching a first surface of the device wafer onto a primary carrier wafer and thinning down the exposed opposite device wafer surface. The bond between the primary carrier wafer and the device wafer is temporary and is removed (i.e., debonded) upon completion of the thinning processing steps. Several temporary bonding techniques have been suggested including the use of adhesive compounds that are chemically dissolved after processing or the use of adhesive tapes or layers that are thermally or via radiation decomposed after processing. Most of these adhesive-based temporary bonding techniques are followed by a thermal slide debonding process where the device wafer and the carrier wafer are held by vacuum chucks while heat is applied to the bonded wafer pair and the wafers slide apart from each other. In the current thermal slide debonding process and the further processing the separated thinned device wafer is held via a secondary support mechanism for further processing. This secondary support mechanism usually adds cost and complications to the processing equipment. It is desirable to reduce the added cost and complications.
Prior art secondary carriers included electrostatic chucking (ESC) and Gel Pak™. Electrostatic chucks use electrostatic attraction to hold the device wafer to a tool chuck face and once charged they are portable. ESC carriers suffer from three primary drawbacks. First the capacitance of the device and its electrostatic effect are diminished with heat. For thermoplastic temporary adhesives, for example, the release mechanism is reheating to soften the adhesive for debonding. During this process the secondary carrier is exposed to higher temperatures which results in limiting adhesion to the secondary carrier. Second, device wafers having topography above 20 micrometers or micro bumps adhere poorly to the secondary ESC chuck. Finally, the development for the electrostatic attractive force is related to the surface area of the ESC chuck and the applied voltage. Although low current is applied, higher voltages are required for smaller wafers below 300 mm resulting in charging up of the device wafer. To date, ESC chucks have been able to be scaled down to 100 mm but not smaller.
Gel Pak™ is a trademarked commercial product designed for temporary secure storage and transfer of diced die and thinned wafers. This method relies on a modified tackified adhesive sheet. A thermoplastic material in nature, this material can be applied to the wafer in sheet form suspended over a support structure which is portable in nature. It uses a vacuum release technology to draw the adhesive sheet tightly against a fabric screen membrane resulting in a reduced contact area with the adhesive and the subsequent release of the wafer. The primary material adhesive sheet however has two primary drawbacks. First, it is affected by increased processing temperature softening and thus releases prematurely before the primary temporary adhesive is released. Second, the material is attacked vigorously by standard solvents used to remove primary thin wafer handling adhesive residue from the device wafer resulting in premature release.
It is desirable to have a portable secondary carrier for thinned wafers that overcomes the above mentioned problems.